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Liu, C, Liu, Y-C, Kao, Y-C, and Shiang, T-Y. Effects of training with dynamic moment of inertia bat on swing performance. J Strength Cond Res 25(11): 2999–3005, 2011—The purpose of this study was to investigate the effects of the 8-week dynamic moment of inertia (DMOI) bat training on swing velocity, batted-ball speed, hitting distance, muscle power, and grip force. The DMOI bat is characterized in that the bat could be swung more easily by reducing the moment of inertia at the initial stage of swing without decreasing the bat weight and has a faster swing velocity and lower muscle activity. Seventeen varsity baseball players were randomly assigned to the DMOI bat training group (n = 9) and the normal bat training group (n = 8). The training protocol was 7 swings each set, 5–8 sets each time, 3 times each week, and 8 weeks' training period. The results showed that the swing training with the DMOI bat for 8 weeks significantly increased swing velocity by about 6.20% (96.86 ± 8.48 vs. 102.82 ± 9.93 km·h−1), hitting distance by about 6.69% (80.06 ± 9.16 vs. 84.99 ± 7.26 m), muscle power of the right arm by about 12.04% (3.34 ± 0.41 vs. 3.74 ± 0.61 m), and muscle power of the left arm by about 8.23% (3.36 ± 0.46 vs. 3.61 ± 0.39 m) (p < 0.05). Furthermore, the DMOI bat training group had a significantly better change percentage in swing velocity, hitting distance, and grip force of the left hand than did the normal bat training group (p < 0.05). The findings suggested that the swing training with the DMOI bat has a positive benefit on swing performance and that the DMOI bat could be used as a new training tool in baseball.

Introduction

Swing velocity is the most important factor to achieve excellent hitting performance in a baseball game (24). Increasing swing velocity is considered one of the approaches to enhance hitting ability. A faster swing velocity reflects more time to judge the pitches, to make a better contact, to increase batted-ball velocity and hitting distance. Bat properties such as weight and moment of inertia are important factors that increase swing velocity. Because the bat swing involves a rotational movement, the moment of inertia of the bat should be taken into consideration. Previous study revealed that swing velocity is affected greatly by the moment of inertia than by bat weight. Therefore, coaches and athletes need to know more about the moment of inertia, which should also be given priority over bat weight when selecting a proper bat (12).

According to the theory of moment of inertia, the distance between distributed mass and the axis of rotation affects the value of moment of inertia. In others words, when the bat mass is less and close to the axis of rotation, the moment of inertia is lower, and it is relatively easy to swing and control the bat. On the other hand, a greater bat mass or higher moment of inertia will generate a greater momentum, which makes it difficult to swing, and the swing velocity is then reduced (15). Usually, the greater the bat mass, the longer and larger the bat becomes; this is why its hitting area gets bigger, whereas the swing velocity gets slower (1). To increase swing velocity, one can reduce the bat mass; however, by reducing the bat mass and, thus, the moment of inertia, the effective mass of the bat will also be decreased, so this makes it more difficult for athletes to hit the sweet spot accurately.

With respect to the above, Hung et al. (15) designed a bat featuring dynamic moment of inertia (DMOI bat). The concept lies in the fact that the mass of the DMOI bat is close to the axis of rotation at the handle during the initial stage of swing, so this would reduce the bat's moment of inertia and make it easy to swing, whereas the distribution of mass changes to be the same as a normal bat in the moment of bat–ball contact, so this decreases the moment of inertia of the initial swing without reducing bat weight. Previous studies showed that a 860-g DMOI bat had the fastest swing velocity, a similar swing trajectory and kinematic data of upper limbs compared with that of the normal bat, which indicates that hitting with a DMOI bat will not change the bat's trajectory and athletes' swing movement while having a faster swing velocity in the swing phase and the moment of bat–ball contact and a lower mean electromyographic magnitude of triceps. The study suggested that the DMOI bat could not only use the dynamic change of moment of inertia while swinging to recruit fewer motor units of upper-limb muscle groups but also have a faster swing velocity without changing one's batting stance. The DMOI bat was considered the choice of training bat for its better swing outcome (17).

Previous studies have revealed that the fast-twitch motor unit would be selectively given priority to the recruitment during faster movements (11,13). Therefore, the hypothesis of this study was that long-term swing training with a DMOI bat may enhance swing performance, which resulted from inducing neuromuscular adaptation. The aim of this study was to examine the effect of DMOI bat training for 8 weeks on swing velocity, batted-ball velocity, hitting distance, explosive force, and grip force to understand if a long-term DMOI bat training was effective in enhancing hitting performance.

Methods

Experimental Approach to the Problem

This study was designed to examine the effects of the 8-week Dynamic Moment of Inertia (DMOI) bat training on swing velocity, batted-ball speed, hitting distance, muscle power, and grip force. The DMOI bat is characterized in that the bat could be swung more easily through reducing the moment of inertia at the initial stage of swing without decreasing the bat weight, as well as has faster swing velocity and lower muscle activity. All subjects recruited from the same baseball team, assigned to the DMOI bat and standard bat training groups and participated in 35–56 dry swings in 24 training sessions during an 8-week training period. The experiment design was repeated measurement. The swing velocity, batted-ball speed, hitting distance, muscle power, and grip force were tested as the same measurement procedure before and after training.

Subjects

Seventeen varsity baseball players who were recruited from the Shih-Hsin University in Taiwan participated in this study. Starters or bench players were randomly assigned to the DMOI bat training group (n = 9) and the normal bat training group (n = 8). All of the subjects had >9 years of baseball playing experience and at least received 3 hours of training session daily, 5 d·wk−1 during the regular training season. The team that subjects were recruited from got the championship in the Division-II of interuniversity baseball tournament in Taiwan before taking part in this study. The physical characteristics of the subjects are described in Table 1. This study was approved by the Human Ethics Committee at Taipei Physical Education College. All subjects in this study were informed of the experimental risks and signed an informed consent before participation.

Training Protocol

The swing training was carried out during the off-season period after the interuniversity baseball tournament in Taiwan. To avoid results being affected by other training, all subjects were asked not to do any regular baseball practice or weight training during the experimental period. Based on a previous study result that an 860-g DMOI bat had the fastest swing velocity than bats weighing 900, 920, and 940 g (17), this study chose 860 g as the weight of the training bat for 2 groups. For the DMOI bat-training group, 3 DMOI bats (weight: 860 g; length: 86 cm) with alloy metal were specially made for this experiment. As shown in Figure 1, the barrel of the DMOI bat was simulated as a slide rail in the same diameter as the handle. A sliding mass in the shape of a hollow tube was wrapped around the slide rail and it would slide toward the barrel end to change the bat's moment of inertia when the bat was swung. The swing training with the DMOI bat is shown in Figure 2. Three wooden baseball bats (weight: 860 g; length: 86 cm; material: Ash; model #: Pro Taiwan-666; manufacturer: Joinsun Corp., Taichung, Taiwan) were used for the normal bat training group. The swing training protocols for 2 groups are shown in Table 2. Both groups performed dry swing after warm-up exercise at the regular batting practice area for 8 weeks, 3 times a week, 5–8 sets each time (1 set increment every 2 weeks), and 7 swings per set (3 strikes and 4 balls on single batting). The number of total swings a week was from 105 swings to 168 swings.

Measurement Procedures

The same measurement procedures were carried out on the same day at pretest and posttest sessions. All subjects did regular warm-up exercise before measurement and were allowed to practice several times for each testing item. A new official provided wooden baseball bats (weight: 860 g; length: 86 cm; material: ash; model #: Pro Taiwan-666; manufacturer: Joinsun Corp.) and several new official game used baseballs (weight: 147.53 g; model #: KY-500, manufacturer: Tayang Sporting Corp., Kaohsiung, Taiwan) were used for the measurements of swing velocity, batted-ball velocity, and hitting distance.

Swing Velocity

The Baseball and Softball Swing Velocity Detection System (Utek Corp., Taipei, Taiwan) was used to measure swing velocity. A batting tee as the subjects' hitting target was placed in front of the infrared shutter of the detection system so that the sweet spot of each batting was able to pass in front of the infrared shutter and the swing radius would be the same. To standardize the test, all subjects were asked to grip the bat handle at the distal end and take on their habitual batting stance. To obtain the swing velocity at the moment of bat–ball contact, each subject was required to adjust his relative position to the batting tee so that the checkpoint of swing velocity was at his best hitting point in the strike zone. Each subject performed 3 hits, and an average value was computed for data analysis.

Hitting Distance

This test was executed in a standard ballpark. Each subject was asked to stand in the batter's box gripping the distal end of the bat handle while a baseball coach dropping his hand did soft tossing to each subject's designated best hitting point. Each subject, in his habitual swing stance, was required to do a full swing 5 times, among which the farthest distance was selected for data analysis.

Batted-Ball Velocity

A multifunctional sports radar gun (Stalker Corp., Model: 1-888-Stalker, Plano, TX, USA) was set up on a 1-m tall tripod, which was 1 m directly behind the home plate. The sports radar gun was pointed to the direction of the pitcher's mound, and the batted-ball velocity was detected and recorded at the same time as the test of hitting distance was performed. Each subject was asked to hit 5 times among which the best result was taken for statistical analysis.

Explosive Force

To get the data of explosive force, subjects were asked to do shotput in a sitting position by using a standard regulation weight (ca. 7.26 kg/16 lb) shot for men's competition. Each subject was asked to sit on a chair pushing the shotput out in full strength with one hand while the other hand was on his waist to avoid affecting the result. This was done 2 times each for the left and right hands, and the best result was taken for statistical analysis.

Grip Force

This test was executed using a grip strength dynamometer (Yagami Corp., Nagoya City, Japan) to measure the maximum grip strength of all subjects' left and right hands. The width of the dynamometer handle was adjusted according to the individual's preference; the ideal width was chosen so that the handle should rest on the middle of 4 fingers. During the test, the subject stood still and upright looking ahead, and the hand to be tested could not be in contact with the thigh. Each hand was tested twice; the best result was taken for statistical analysis.

Results

Table 3 presents the ICCs for all the dependent variables. The high coefficients (0.618–0.832) indicate the test–retest reliabilities are good to excellent for all the variables measured in this study.

In addition, swing training with the DMOI bat compared to that with the normal bat has a significantly greater change percentage in swing velocity by 6.20%, hitting distance by 6.69%, and grip force of the left hand by 7.75%, respectively (p < 0.05). The batted-ball velocity, both arm of explosive force, and right hand of grip force, were not significantly different between groups (p > 0.05). Table 4 shows the training effects of 2 groups for the 8 weeks' training period.

Discussion

The findings indicated that the 8-week DMOI bat training did produce positive effects in increasing the swing velocity, hitting distance, and left and right arm explosive forces. Moreover, the DMOI bat training group has a better change percentage in swing velocity, hitting distance, and grip force of the left hand than the normal bat training group did. Previous studies showed that there was a significant difference in swing velocity after taking long-term training with a weighted bat, light bat, and normal bat (5,8,21). Sergo and Boatwright (21) conducted the swing training with 24 college baseball players who were randomly given a normal bat and a weighted bat (62 oz.) alternating with a coach bat (lighter than the normal bat) and weighted bat in a 6-week training period, 3 d·wk−1, 100 dry swings each day. The results indicated that there was a significant increase in swing velocity for all 3 groups, but no significant difference was found among the groups, showing that the training effect was pretty much the same among 3 groups. Furthermore, in a study investigating the immediate effect of switching between different bat weights, the results showed that if the subject first swung with a light bat and then immediately switched to a normal bat, the increased swing velocity was significantly higher than the first swinging with a normal bat or a weighted bat (6). Southard and Groomer (22) further examined the effect of warming up with different moment of inertia bats on bat velocity and swing pattern; the finding showed that the lowest swing velocity was with the greatest moment of inertia bat. In view of this, the swing velocity might play a decisive role during swing training on affecting swing performance. Therefore, the findings of this study on the 6.20% increase of swing velocity by using the DMOI bat for 8 weeks not only explained the reason why the hitting distance also increased by about 6.69% but they also indicated that the DMOI bat, whose moment of inertia increased along with the motion of bat swinging, gave a better training outcome in swing training.

Baseball swing is considered as a kind of ballistic movement. Therefore, players need to take specific resistance training to be able to have a faster swing (9,23). The so-called specific training should include overload training, resistance training within the range of motion, actual motion in acceleration pattern, and safety (10). Weatherly and Schinck (25) believed that long duration, heavy-weighted, and nonspecific weight training would decrease the speed of motion. Normally, players prefer using a heavier bat in swing training; by doing so, more motor units can be stimulated, and the effect of training will be revealed when switching back to the normal bat (7). Hung et al. (16) once investigated on the proper bat weight from the perspective of muscle activation. The result showed that the triceps were the main functional muscle groups and that the integrated electromyographic (IEMG) and mean electromyographic (MEMG) of triceps and biceps tended to decrease along with the decrease in bat weights. However, hitting with a heavier bat will reduce the swing velocity (2,12,18,19).

On the other hand, when hitting with a lighter bat, the swing velocity is relatively higher (18), but the stimulation to muscle groups is slightly insufficient. A series of previous studies proved that, with the same bat weight, the DMOI bat and the normal bat are similar in swing trajectory and upper-limb joint angular changes plus the DMOI bat having faster swing velocity (17), indicating that the DMOI bat can have a faster swing velocity under a similar muscle group stimulation. Cross and Bower (4) also found that, with the same mass but different swing weights (as moment of inertia), the swing velocity of the softball bat and golf club increases along with the decrease of swing weight. Because the DMOI bat has the same mass as the normal bat in this study, its moment of inertia would become greater along with the swing motion; therefore, hitting with a DMOI bat could give enough stimulation to the relative muscle groups, and so it could have faster swing velocity.

Because batting involves a rapid ballistic movement, Newton et al. (20) investigated the differences between upper-body ballistic movements and traditional bench press movements on the average power, average explosive power, and peak explosive power. The results showed that the ballistic throwing movement has a faster speed of movement in addition to generating greater concentric contraction force. During high speed movement, the high-threshold fast-twitch motor unit that innervates type 2 muscle will be selectively given priority to activation (11,13). The explosive training with different bat weights would result in neurological adaptation, enhance the recruitment of motor units, increase the stimulation frequency of motor nerve, improve motor unit synchronization, and remove neural inhibition (25). In view of the above findings, a long-term training with a DMOI bat can enhance swing velocity and upper-limb explosive force for its being able to give priority to recruit fast-twitch motor units that can generate greater tension within a short time; as a result, the hitting distance would also be improved.

This result of the study showed that the DMOI bat training did not improve the batted-ball velocity. The batted-ball velocity was affected by factors such as bat swing velocity, pitch speed, the ball's coefficient of restitution, the bat's flexural properties (trampoline effect) and curvature, and the collision spot of the ball on the bat (14). Given the fact that this study was conducted by using the same bat, hitting with the balls of the same brand, and with the same pitch speed, the batted-ball velocity not being able to have a significant increase was suspected to be partially because of the differences of bat–ball collision spots and could be partially contributed to the sports radar gun used for speed detection in this study. Because the radar gun uses radar to detect the moving speed of the ball, it might not be as precise in detecting the batted-ball velocity at a very close distance. Nevertheless, the radar gun was the only device available for this study, and it was considered as a limitation of this study.

As far as the bat was concerned, it would be easier to swing and control with a lower moment of inertia when the bat mass was low and distributed near the axis of rotation. In view of this, training with a lighter bat could have the effect of nerve adaptation because of a fast-twitch motor unit being induced with priority. Contrarily, a heavier bat or a bat with a greater moment of inertia would generate a greater momentum, which decreased the swing velocity for being difficult to swing. But training with a weighted bat would have a positive effect on muscles because the heavier weight could give the muscles greater stimulation, just like giving sport-specific resistance training. The objective of this study—the DMOI bat—combines the features of a light bat and a weighted bat in a way that the bat mass is distributed near the axis of rotation around the bat handle during the initial swing period, so that it would be easy to swing with a decrease in the moment of inertia. While in the moment of bat–ball collision the mass distribution of the DMOI bat was the same as that of a normal bat, so that the moment of inertia could be maintained in lower level since the initial swing without decreasing bat weight. Previous study already proved that the DMOI bat, with batting stance remaining unchanged, can use the dynamic changes of the bat's moment of inertia to recruit fewer motor units of the upper-limb muscles and, therefore, have a faster swing velocity (25). This study further discovered, from the training aspect, that an 8-week DMOI bat training could significantly increase swing velocity, hitting distance, and explosive force and has a greater training effect on swing velocity, hitting distance, and grip force than swing training with a normal bat. Therefore, the DMOI bat could be a new training bat with a positive effect on baseball and softball.

Practical Applications

In the baseball field, players often swing with underweight or overweight bats for increasing swing velocity. However, previous studies have found that swing velocity is affected greatly by the moment of inertia than by bat mass. This study reveals that an 8-week dry swing training with the DMOI bat improved swing performance compared to with a normal bat. The major characteristic of the DMOI bat is that the moment of inertia varies from lower to standard with swing phases. Therefore, the DMOI bat could be considered a new training bat with a positive effect for swing in baseball and softball. We would suggest that baseball and softball players practice with the DMOI bat for long-term training, which can enhance swing velocity and upper-limb explosive force that can improve the hitting distance. The DMOI bat can also be used on deck circles before batting for improving hitting performance. Meanwhile, the concept of the DMOI could even be applied to the training equipment design of other swing sports such as golf, tennis, squash, badminton, for an alternative training tool.